Building an RPT Between RP and Receivers

The RPT is the path between the RP and receivers (hosts) in a multicast group (see Figure 1). The RPT is built by means of a PIM join message from a receiver's DR:

1. A receiver sends a request to join group (G) in an Internet Group Management Protocol (IGMP) host membership report. A PIM sparse-mode router, the receiver's DR, receives the report on a directly attached subnet and creates an RPT branch for the multicast group of interest.
2. The receiver's DR sends a PIM join message to its RPF neighbor, the next-hop address in the RPF table, or the unicast routing table.
3. The PIM join message travels up the tree, and is multicast to the ALL-PIM-ROUTERS group (224.0.0.13). Each router in the tree finds its RPF neighbor by using either the RPF table or the unicast routing table. This is done until the message reaches the RP and forms the RPT. Routers along the path set up the multicast forwarding state to forward requested multicast traffic back down the RPT to the receiver.